[rbp/omxplayer] Avoid flushing fifos when audio/video fifos out of sync

This was intended to be a never happens event that could reset the state when something bad happens.
However it does trigger in some reasonable cases (like audio codec change) and makes things worse.
Merge pull request #3068 from popcornmix/avoid_flush_sync

[rbp/omxplayer] Avoid flushing fifos when audio/video fifos out of sync